This invention relates generally to a cooling technique for a nuclear reactor installation, and more particularly to techniques suited for cooling and submerging a reactor core in the event of a coolant loss accident, and for removing decay heat produced at the reactor core, and for suppressing the increase of pressure within a containment vessel.
In a convectional boiling reactor having an electric output of up to 1,100 MW, a large amount of steam, produced from a ruptured portion at the time of a coolant loss accident, is led into a pressure suppression pool, provided below a reactor pressure vessel, and is condensed there, thereby suppressing the increase of pressure within a primary containment vessel to below an allowable level. Then, an emergency core cooling system (ECCS), which comprises a high-pressure core spray system, a low-pressure core spray system, a low-pressure pouring system and an automatic depressurization system, is operated to pump up the water of the suppression pool to cool the reactor core. At this time, a residual heat removal system feeds, by a pump, the water of the suppression pool to a heat exchanger disposed outside of the containment vessel, thereby removing decay heat from the reactor core.
On the other hand, in a small- to a medium-size boiling reactor having an electric output of up to 600 MW, in order to simplify the installation and to achieve a high safety, it has been proposed that an emergency core cooling system excludes the use of powered equipment, such as a pump, and instead employs a dual accumulator pouring system using a passive method in which gas pressure is beforehand applied to a water reservoir for pouring water to the reactor core under a pressure differential between the water reservoir and the reactor core so as to cool the reactor core upon emergency. Thus, the system heretofore used in the conventional reactor is omitted.
Also, with respect to a small- to a medium-size reactor, Japanese Patent Unexamined Publication No. 63-191096 discloses a system in which the decay heat during a long cooling period after a coolant loss accident is removed by a passive method using a natural force. More specifically, an outer pool is provided around a primary containment vessel, and by utilizing a natural convection of a pressure suppression pool and the outer pool, with the surface of the containment vessel used as a heat transfer surface, the heat is transferred to the outer pool due to a temperature difference between the two pools so as to evaporate the pool water to thereby achieve the cooling.
As described above, the containment vessel in the small- to the medium-size reactor is of such a construction that the steam of high temperature and pressure which has leaked into the primary containment vessel, is led into a pressure suppression chamber in the primary containment vessel so that the steam of high temperature and pressure is condensed by the pressure suppression pool within this chamber.
The system used in the large-size reactor, along the prior art, requires auxiliary powered equipment, including a pump, a heat exchanger and an emergency power source, in order to cool the reactor core and also to remove decay heat produced at the core at the time of a coolant loss accident. Therefore, the construction of the plant becomes complicated, which poses a problem that attention is required so as not to lower the reliability when, for example, the power source is subjected to a malfunction.
On the other hand, if the accumulator pouring system and the outer pool use as the safety equipment for the small- to the medium-size reactor, are adopted to the large-size reactor, the construction of the plant of the large-size reactor can be simplified; however, if they are merely adopted, the size of the primary containment vessel for accommodating the reactor pressure vessel and the pressure suppression chamber becomes excessively large in order to increase the area of heat radiation to the water of the outer pool so as to cope with a large power output.